ADLINK PCI-8554 - Manual (Page 33)

Getting Started •23
Even though it is a dual interrupt system, only one interrupt level is ever
used. The card uses the INT #A interrupt request signal on the PCI bus.
The motherboards circuits will then transfer INT #A to one of the AT bus
IRQ levels. The IRQ level is set by the PCI plug and play BIOS and is
saved in the PCI controller. It is not necessary for users to set the IRQ
level. Users can however read back the IRQ level setting by using the
software library. Refer to the section 5.4.
The PCI controller of cPCI/PCI-8554/R can receive two hardware IRQ
sources. However, a PCI controller can generate only one IRQ to the PCI
bus, the two IRQ sources are distinguished by the ISR software. If both
IRQ are in use, the application software will invoke the
“_8554_GET_IRQ_Status” function to distinguish which interrupt is
inserted and servicing that IRQ. Users must then clear the current IRQ to
allow the next IRQ be processed.
If the application needs only one IRQ, the other IRQ source can be
disabled by software. If your application does not require any IRQ sources,
you can disable both interrupts. However, the PCI BIOS will still assign an
IRQ level to the PCI card and hence will still occupy the PC’s resources if
you only disable the IRQ sources without changing the initial conditions of
the PCI controller. It is however, not recommended that the user re-design
the initial condition of the PCI card with the users’ own application software.
If users want to disable the IRQ level, user can use the ADLINK utility
‘INIT8554.EXE’ to change the interrupt settings.
2.15 Digital Input and Output
To program the digital I/O operation is fairly straightforward. The digital
input operation is used for reading data from its corresponding registers,
and the digital output operation is used for writing data to its corresponding
registers. The digital I/O registers‘ are discussed in section 3.4 and 3.5.